systemd: Start varnishd after nss-lookup.target

Otherwise host resolution may not be available during boot resulting in
startup failure of varnish.service for no good reason.

systemd/varnish.service

 [Unit]
 Description=Varnish Cache, a high-performance HTTP accelerator
-After=network-online.target
+After=network-online.target nss-lookup.target
 
 [Service]
 Type=forking